It is necessary for a semiconductor manufacturing apparatus for heat-treating wafers to control the temperature difference on the surface of the wafer as small as possible (e.g., within a range of plus or minus one degree Celsius) to avoid thermal strain and others. In addition, it is necessary to elevate the temperature fast (e.g., 100° C./s) to the desired temperature (e.g., 1350° C.). Accordingly, an induction heating device is widely known in which an induction heating coil is divided into a plurality thereof and a power control is performed by connecting a high-frequency power source (e.g., an inverter) to each of the divided induction heating coils individually. However, as each of the divided induction heating coils is disposed closely to each other, there are mutual induction inductances M, thereby generating mutual induction voltages. Therefore, each of the inverters is operated in parallel via a mutual inductance and it may cause a mutual power transfer between the inverters when having a mutual phase shift of electric current between the inverters. In other words, as a phase shift occurs in a magnetic field between the divided induction heating coils due to a phase shift of electric current between each of the inverters, magnetic fields in the vicinity of the boundary of the adjacent induction heating coils are weakened, thereby reducing the density of heat generated by an induction heating power. As a result, temperature variations may occur on the surface of the heated object (such as a wafer).
This prompted inventors and others to propose a technique of Zone Controlled Induction Heating (ZCIH) (e.g., refer to Japanese Patent Application Publication No. JP2007-026750A) to enable to appropriately control the induction heating power, by preventing a circulation current from mutually flowing between the inverters even under a situation where a mutual inductance exists due to a mutual induction voltage between the adjacent induction heating coils, as well as preventing heat density from degrading in the vicinity of the boundary of the divided induction heating coils. According to the technique of ZCIH, each power supply unit is provided with a step-down chopper and a voltage source inverter (hereinafter referred to simply an inverter). Then, each of the power supply units divided into a plurality of power supply zones is individually connected to each of the induction heating coils, respectively, for supplying power.
In this case, each of the inverters in each of the power supply units are controlled for synchronization of current (i.e., synchronization control of current phase), respectively, and by synchronizing phase of a current flowing in each of the inverters, a circulation current is prevented from flowing among the plurality of inverters. In other words, by suppressing electric current among the plurality of inverters, an overvoltage generated by the regenerative electric power flowing to the inverter is avoided. In addition, by synchronizing the current phase flowing in each of the divided induction heating coils, heat density by induction heating power is intended not to be degraded rapidly in the vicinity of the boundary of each of the induction heating coils. Furthermore, by varying input voltage of each of the inverters, each of the step-down choppers controls the current amplitude of each of the inverters, thereby controlling the induction heating power supplied to each of the induction heating coils. That is, in a technique of ZCIH disclosed in Japanese Patent Application Publication No. JP2007-026750A, by performing current amplitude control for each step-down chopper, power of the induction heating coil is controlled in each zone, and by controlling synchronization of current phase of each inverter, circulating current among the plurality of inverters is suppressed and the density of the heat generated by the induction heating power is homogenized in the vicinity of the boundary of each of the induction heating coils. By the control system for the step-down chopper and the control system for the inverter performing control individually using such a ZCIH technique, it is possible to control heat distribution on the object to be heated as desired. That is, it is possible to perform rapid and precise temperature control and heat distribution control, using the ZCIH technique disclosed in Japanese Patent Application Publication No. JP2007-026750A.
Further, a technique is disclosed in Japanese Patent Application Publication No. JP2004-146283A for supplying DC power at the same time to each of inverters connected, respectively, to each of a plurality of induction heating coils, thereby running a plurality of induction heating coils concurrently. Specifically, this technique is adapted to detect the zero crossing of the output current from each of inverters connected to each of series resonant circuits, respectively, to compare the zero-cross timing of the output current of each of the inverters and the rising timing of the reference pulse. This technique is intended to synchronize the output current from each of the inverters, by adjusting the frequency of the output current so that a phase shift from the reference pulse, calculated individually by comparison, becomes zero or close to zero. In addition, this technique controls the current flowing through the induction heating coil after the output current of each inverter is synchronized, by increasing and decreasing the output voltage of the inverter, and conducts homogenized heat distribution of the object to be heated.
Chapter 8 Resonance Type Converter Circuit, in “Power Electronics Circuit” compiled by Expert Committee on Semiconductor Power Conversion System Investigation, the Institute of Electrical Engineers of Japan (IEEJ) and published by Ohmsha, describes a resonance type converter circuit that has a resonant current phase lag mode, in which the phase of the output current from the inverter is behind relative to the output voltage of the inverter, and a resonant current phase lead mode, in which the phase of the output current from the inverter is ahead relative to the output voltage of the inverter. It describes that a resonance type converter circuit in a resonant current phase lead mode is turned on in the zero-current switching, and that a reverse recovery current of a commutation diode is added to the resonant current as a current flowing through the switching element, due to the reverse recovery operation of the commutation diode at turn-on of the switching element, which results in the increase in turn-on power loss of the switching element. It also describes that a resonance type converter circuit in a resonant current phase lag mode, on the other hand, becomes zero-current switching at a turn-on operation and hard switching at a turn-off operation, where a turn-off operation in hard switching can be improved as Zero Voltage Switching (ZVS), by connecting a lossless capacitor snubber in parallel with the switching element.
Further, “Transistor Gijutsu”, CQ Publishing, June 2004 Issue, p. 228, discloses a full bridge circuit that achieves a ZVS operation to stably drive an inductance load, by shunting an output at zero-crossing of a current thus avoiding a state that a switching element becomes open.